High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers.
For certain applications, such as injection or extrusion, and blow molding or blown film extrusions, it is desirable to use polyester resins of high and stable melt viscosity. Linear polyesters, however, are known to interact with aromatic carbonates and (poly-)carbonates during compounding to give very high molecular weight/melt viscosity products.
Such carbonate species may include, e.g., decabromodiphenyl carbonate, copolycarbonates of bisphenol-A and tetrabromobisphenol-A, bisphenol-A polycarbonate, tetrabromobisphenol polycarbonate, and the like. Increase in melt viscosity is particularly undesirable when the polyester is compounded with bisphenol-A (BPA)-tetrabromo BPA copolycarbonate (flame retardant), and Sb.sub.2 O.sub.3 (synergistic flame retardant additive) that, unavoidably, also acts as catalyst in the linear polyester-polycarbonate reaction. See, U.S. Pat. Nos. 3,936,400; 3,833,685; 3,833,535; 3,855,277; and 3,915,926. The interaction causing increases in melt viscosity proceeds via terminal OH groups on the polyester, e.g., poly(1,4-butylene terephthalate), molecules. It has now been discovered that by blocking these with monofunctional reagents of low volatility, so that they survive the polyester manufacturing process, the interaction with aromatic carbonate is blocked and melt viscosity of the blends can be kept under control.